| 1. |
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| 2. |
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| 3. |
- Leijon, Mats, et al.
(författare)
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An electrical approach to wave energy conversion
- 2006
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Ingår i: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; :31, s. 1309-1319
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Tidskriftsartikel (refereegranskat)abstract
- Motions in nature, for example ocean waves, can play a significant role in tomorrow's electricity production, but the constructions require adaptations to its media. Engineers planning hydropower plants have always taken natural conditions, such as fall height, speed of flow, and geometry, as basic design parameters and constraints in the design. The present paper describes a novel approach for electric power conversion of the vast ocean wave energy. The suggested linear electric energy converter is adapted to the natural wave motion using straightforward technology. Extensive simulations of the wave energy concept are presented, along with results from the experimental setup of a multisided permanent magnet linear generator. The prototype is designed through systematic electromagnetic field calculations. The experimental results are used for the verification of measurements in the design process of future full-scale direct wave energy converters. The present paper, describes the energy conversion concept from a system perspective, and also discusses the economical and some environmental considerations for the project.
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| 4. |
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| 5. |
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| 6. |
- Majdi, Saman, 1977-, et al.
(författare)
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Enhanced Hall mobility in graphene-on-electronic-grade diamond
- 2023
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 123:1
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Tidskriftsartikel (refereegranskat)abstract
- The outstanding electronic properties of graphene make this material a candidate for many applications, for instance, ultra-fast transistors. However, self-heating and especially the detrimental influence of available supporting substrates have impeded progress in this field. In this study, we fabricate graphene-diamond heterostructures by transferring graphene to an ultra-pure single-crystalline diamond substrate. Hall-effect measurements were conducted at 80 to 300 K on graphene Hall bars to investigate the charge transport properties in these devices. Enhanced hole mobility of 2750 cm(2) V-1 s(-1) could be observed at room-temperature when using diamond with reduced nitrogen (N-s(0)) impurity concentration. In addition, by electrostatically varying the carrier concentration, an upper limit for mobility is determined in the devices. The results are promising for enabling carbon-carbon (C-C) devices for room-temperature applications.
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| 7. |
- Ahlqvist, Jan, et al.
(författare)
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Bone demarcation of the temporomandibular joint. Validity of clinical assessment of bone thickness by means of CT.
- 1998
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Ingår i: Acta Radiologica. - 0284-1851 .- 1600-0455. ; 39:6, s. 649-55
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: To study the CT depiction of bone demarcations in the temporomandibular joint, using conventional window level and window width; and to evaluate observer performance in estimating bone thickness in these images.MATERIAL AND METHODS: Seven joint specimens were imaged by CT and then cryosectioned. The measurements of bone wall thickness in the images were compared to the true bone thickness at each cutting level. In addition, 4 experienced radiologists estimated the thickness of the bone walls in the images.RESULTS: The relative difference between the CT reproduction and the true bone thickness was small for bone walls thicker than 2 mm. This difference increased with the decrease in bone thickness and the increase in the inclination of the bone wall from the perpendicular to the image plane. Bone walls thinner than 1 mm were reproduced as considerably thicker than their true thickness. This resulted in a clinical overestimation of bone thickness.CONCLUSION: Both the CT representation and the interpretation of bone demarcation in the temporomandibular joint may constitute a problem. Partial volume averaging effects can result in an overestimation of bone dimensions amounting to 200% for thin bones. The central white zone in images of thin bone walls obtained with the parameters described here could serve as an indicator that could help to reduce the risk of overestimating bone thickness.
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| 8. |
- Ahlqvist, Jan, et al.
(författare)
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Sources of radiographic distortion in conventional and computed tomography of the temporal bone.
- 1998
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Ingår i: Dento-Maxillo-Facial Radiology. - 0250-832X .- 1476-542X. ; 27:6, s. 351-7
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVES: To identify those bony regions of the glenoid fossa where, due to the inclination, there is an increased risk of radiographic distortion in conventional and computed tomography (CT).METHODS: The inclination of the roof and posterior wall of the glenoid fossa was determined relative to established imaging planes. Measurements were performed on 50 corrected coronal MR and 50 axial CT images and 200 sagittal cryosections of 50 temporomandibular joints (TMJs). The location of regions with unfavourable bone wall inclination was identified using the condyle as a reference.RESULTS: The inclination of parts of the fossa roof exceeded the limit for reliable depiction in corrected sagittal and coronal planes in 40% and 8% of the joints respectively. The inclination of parts of the posterior wall of the fossa exceeded the limit for reliable depiction in corrected sagittal and in true sagittal planes in 100% and 84% of the joints respectively. In 84% of the joints the inclination exceeded the limit for reliable depiction in the axial plane. For both bone walls the regions with unfavourable inclination were in the medial part of the joint.CONCLUSIONS: The angulation of parts of the roof and posterior wall of the glenoid fossa in relation to established imaging planes makes them highly susceptible to distortion. The oblique coronal projection is well suited for depiction of the roof of the fossa and preferable to a sagittal projection. An oblique axial projection is required for the posterior wall.
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| 9. |
- Ahlqvist, Jan, 1952-
(författare)
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The temporomandibular joint : Tomopraphic and CT assesment of its bone demarcations with reference to adjacent organs
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The wall of the temporal bone separating the temporomandibular joint (TMJ) from surroundings organs, can be very thin and also have development defects. Distortion effects in the radiographic reproduction of these bone walls can result in misinterpretations when exanimating suspected pathologic changes in the region. These areas need to be radiographic identified prior to taking any invasive measures. Incorrect assessment of bone thickness may lead to serious sequelae due to the risk of penetration injury during invasive investigation or treatment of the TMJ or ear. The purpose of this project was to gain more detailed knowledge about the anatomy and topography of the TMJ with special reference to its bone demarcations regarding adjacent organs and to evaluate the tomographic and computed tomographic (CT) depiction of these bone walls. To obtain a basic analysis of the tissue anatomy and tomographic and CT reproduction of the TMJ region, autopsy specimens were studied. After CT and conventional tomography, the specimens were sectioned in a microtom. Three- dimensional orientation systems allowed identification of section depth in the radiograms and in the histologic sections, allowing the radiograms in turn to be correlated with the true anatomy. The angle of inclination relative to the perpendicular to established imaging planes the bone walls studied was examined in three projections in order to identify regions where the bone demarcation showed an unfavorable inclination regarding the possibility of valid radiographic representation. The thickness of the bone wall between the TMJ and the middle cranial fossa, measured in the thinnest part, varied between 0.08 and 3.62 mm, averaging 1.14 mm. The bone wall between the TMJ and the middle ear showed less variation in thickness ranging from 0.00 to 1.80 mm. The thickness of the bone wall separating the TMJ from the external auditory canal varied between 1.50 mm (lateral part) and 1.21 mm (central part), with a range of between 0.21 and 4.10 mm. Development defects of this bone wall were found in 5.2 % of the examined joints. The validity in tomographic depiction of these walls was highly dependent on an optimal orientation of the bone wall in relation to the image plane. The variations in the anatomy and sagittal dimension of the external auditory canal led to variations in tomographic blurring, and suggested the need for examinations after patient repositioning in cases of suspected bone resorbing lesions so that image aberration due to unfavorable inclination of the bone wall relative to the image plane may be excluded. CT of these bone walls was valid (± 10 %) for walls thicker than approximately 1 mm, forming an angle of less than 35® with the perpendicular to the scan plane when the bone wall thickness was determined as the full-width-at-half-maximum (FWHM). For bone walls thinner than 1 mm, and for those thicker than 1 mm and at an angle exceeding 35®, partial volume averaging effects resulted in a progressively increasing magnification of bone dimensions. Observer estimations of bone thickness from images obtained using conventional bone window settings (c=400, W=2000) showed good agreement for bone walls thicker than 1 mm and with an angle of inclination relative to the perpendicular to the image plane of less than approximately 25®. For bone walls thinner than 1 mm and for thicker than 1 mm with an inclination exceeding approximately 25®, the estimations resulted in a progressively increasing overestimation amounting 200% for gracile bone walls with an inclination of 45® to 50®. Determination of width or absence of the central white zone in images obtained with the described parameters could help to reduce the risk of overestimation of bone thickness. A considerable part of the bone walls separating the TMJ from the middle cranial fossa and the external auditory canal/middle ear, respectively, have dimensions and inclinations to established imaging planes used at TMJ examinations that make the depiction of these walls highly susceptible to image distortion.
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| 10. |
- Ahlqvist, Jan, et al.
(författare)
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Validity of computed tomography in imaging thin walls of the temporal bone
- 1999
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Ingår i: Dento-Maxillo-Facial Radiology. - : British Institute of Radiology. - 0250-832X .- 1476-542X. ; 28:1, s. 13-19
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVES: To evaluate the validity of computed tomography (CT) for reproduction of the bone margins of the temporomandibular joint (TMJ).METHODS: Seven TMJ specimens were examined with a CT and then cryosectioned. The bone separating the TMJ from the middle cranial fossa, middle ear and external auditory canal was measured as the full width at half maximum (FWHM). Measurements were compared with the true thickness of the bone wall.RESULTS: There was good agreement when the bone walls were thicker than 1 mm: accuracy was influenced only by the angle of the bone wall to the scanning plane. Conversely, bone walls thinner than 1 mm were reproduced with a magnification that increased with decreasing bone thickness. The difference increased further as the inclination of the bone wall became greater.CONCLUSION: Measurements performed at FWHM are reliable within +/- 10% for bone walls more than approximately 1 mm thick which form an angle of less than 35 degrees to the perpendicular of the scanning plane. For bone walls thinner than 1 mm and for those thicker than 1 mm with an inclination exceeding approximately 35 degrees, partial volume effects result in a progressively increasing magnification of bone thickness.
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| 11. |
- Andersson, Joakim, et al.
(författare)
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Diamanter blir var mans egendom
- 2003
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Ingår i: Forskning & Framsteg. ; :1, s. 38-41
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 12. |
- Asad, M., et al.
(författare)
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Graphene FET on Diamond for High-Frequency Electronics
- 2022
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Ingår i: IEEE Electron Device Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 0741-3106 .- 1558-0563. ; 43:2, s. 300-303
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Tidskriftsartikel (refereegranskat)abstract
- Transistors operating at high frequencies are the basic building blocks of millimeter-wave communication and sensor systems. The high charge-carrier mobility and saturation velocity in graphene can open way for ultra-fast field-effect transistors with a performance even better than what can be achieved with III-V-based semiconductors. However, the progress of high-speed graphene transistors has been hampered by fabrication issues, influence of adjacent materials, and self-heating effects. Here, we report on the improved performance of graphene field-effect transistors (GFETs) obtained by using a diamond substrate. An extrinsic maximum frequency of oscillation fmax of up to 54 GHz was obtained for a gate length of 500 nm. Furthermore, the high thermal conductivity of diamond provides an efficient heat-sink, and the relatively high optical phonon energy of diamond contributes to an increased charge-carrier saturation velocity in the graphene channel. Moreover, we show that GFETs on diamond exhibit excellent scaling behavior for different gate lengths. These results promise that the GFET-on-diamond technology has the potential of reaching sub-terahertz frequency performance.
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| 13. |
- Balmer, Richard S., et al.
(författare)
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Transport behavior of holes in boron delta-doped diamond structures
- 2013
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:3, s. 033702-
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Tidskriftsartikel (refereegranskat)abstract
- Boron delta-doped diamond structures have been synthesized using microwave plasma chemical vapor deposition and fabricated into FET and gated Hall bar devices for assessment of the electrical characteristics. A detailed study of variable temperature Hall, conductivity, and field-effect mobility measurements was completed. This was supported by Schrodinger-Poisson and relaxation time calculations based upon application of Fermi's golden rule. A two carrier-type model was developed with an activation energy of similar to 0.2 eV between the delta layer lowest subband with mobility similar to 1 cm(2)/Vs and the bulk valence band with high mobility. This new understanding of the transport of holes in such boron delta-doped structures has shown that although Hall mobility as high as 900 cm(2)/Vs was measured at room temperature, this dramatically overstates the actual useful performance of the device.
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| 14. |
- Balmer, R. S., et al.
(författare)
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Unlocking diamond's potential as an electronic material
- 2008
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Ingår i: Philosophical Transactions. Series A. - London : Royal Society of London. - 1364-503X .- 1471-2962. ; 366:1863, s. 251-265
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we review the suitability of diamond as a semiconductor material for high-performance electronic applications. The current status of the manufacture of synthetic diamond is reviewed and assessed. In particular, we consider the quality of intrinsic material now available and the challenges in making doped structures suitable for practical devices. Two practical applications are considered in detail. First, the development of high-voltage switches capable of switching voltages in excess of 10kV. Second, the development of diamond MESFETs for high-frequency and high-power applications. Here device data are reported showing a current density of more than 30mAmm -1 along with small-signal RF measurements demonstrating gigahertz operation. We conclude by considering the remaining challenges which will need to be overcome if commercially attractive diamond electronic devices are to be manufactured.
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| 15. |
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| 16. |
- Bodin, Jan, 1964-, et al.
(författare)
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Customer involvement in a technical product development process : time to implement a service-dominant logic perspective?
- 2011
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Ingår i: The customer is NOT always right?. - 0939783150 ; , s. 331-
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Konferensbidrag (refereegranskat)abstract
- In mainstream marketing the full potential of customer involvement is still largely over-looked. Value is mainly seen as created by the firm, built into products and delivered or marketed to customers. Many product development processes reflect this by emphasizing in-house centered processes possible to monitor and control by management. Today, customers are to a higher extent involved throughout the development process and companies are starting to explore the benefits of opening up the in-house process and involve customers in a more dynamic way. However, it could be argued that the customer is still seen as yet another resource that should be utilized in an optimal way. In contrast to this in-house product development process is the service-dominant logic (S-D logic), which views customers as co-creators of value with the firm. This paper examines the mainstream logic of customer involvement in the product development process through the lens of S-D logic. A comparison between the two logics is made by highlighting the differences regarding: the role of the firm, opportunity instigator, drivers of development, knowledge & skills, role of customers, and role of management. Via a case from the automotive industry focusing on innovation diffusion problems, issues relating to immaterial rights, what happens when the client under-value the partner’s know-how, and therefore is incorrect in their assessment and decisions are highlighted. The paper concludes by discussing both benefits and problems with implementing S-D logic in the product development process.
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| 17. |
- Djurberg, Viktor, et al.
(författare)
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Determination of the acoustic phonon deformation potentials in diamond
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:4
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between acoustic phonons and electrons in diamond has been investigated by comparing state-of-the-art time-of-flight drift velocity measurements with Monte Carlo simulations. We use a multivariable anisotropic description of acoustic deformation potential scattering. The phonon-electron interaction is the limiting factor for the carrier mobility in ultrapure single crystal diamond. Hence, having a correct description is necessary for both device simulations and for predicting the maximum device performance. The experiments were performed at low temperature and using ultrapure diamond to minimize the influence of other scattering sources. The electronic valley polarization in diamond at low temperatures enables determination of both uniaxial and dilatation deformation potentials in the same experiment. The uniaxial and dilatation deformation potentials are found to be 18.5±0.2 and −5.7±0.3 eV, respectively.
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| 18. |
- Djurberg, Viktor, et al.
(författare)
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Investigation of Photoexcitation Energy Impact on Electron Mobility in Single Crystalline CdTe
- 2021
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 14:15
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Tidskriftsartikel (refereegranskat)abstract
- The exceptional electronic properties of cadmium telluride (CdTe) allow the material to be used in a wide range of high energy radiation detection applications. Understanding the mechanisms of local carrier scattering is of fundamental importance to understand the charge transport in the material. Here, we investigate the effect of photoexcitation on electron transport properties in chlorine doped single crystalline cadmium telluride (SC-CdTe:Cl). For this purpose time of flight measurements were performed on SC-CdTe:Cl in order to study the electron drift mobility in the low injection regime. Measurements were made at the temperature intervals of 80 to 300 K, for an applied electric field between 270 and 1600 V/cm and for wavelengths of 532, 355 and 213 nm. We have found that the electron drift mobility was affected by the excitation energy for temperatures below 200 K. In addition, the measurements revealed that it is possible to determine impurity and shallow trap concentration by this method. The method proves to be extremely sensitive in measuring very low impurity levels and in identifying dominant scattering mechanisms.
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| 19. |
- Djurberg, Viktor, 1993-
(författare)
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Low Temperature Charge Transport in Diamond
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Diamond is a wide band semiconductor with fascinating electrical and physical properties. It has high thermal and electrical conductivity, high electrical breakdown field, high radiation hardness and is chemically inert. These properties make diamond an excellent material for high power electronics, high frequency electronics, particle detectors and for electronics in hazardous environments. Moreover, diamond has been suggested for applications in valleytronics.Valleytronics is a term for semiconductor technology that exploits minima in an energy band, so called valleys. In diamond there are six of these valleys in the conduction band and the conduction electrons resides in one of these six valleys at low temperatures. The valley an electron is in, its valley polarization, affects how it behaves in an electric field. The valley polarization along with an understanding of the electron-phonon scattering processes makes a good framework for understanding of electron transport in diamond. In this thesis, both of these topics have been explored, with the purpose of understanding low temperature electron transport in diamond. A detailed description of low temperature charge transport is relevant for several reasons. Firstly, it can help with understanding the charge transport in e.g. detectors. Secondly, it gives more degrees of freedom when designing new electronics. In this thesis, both experiments and simulations has been used investigate low temperature transport in diamond. The main experiment method used was time-of-flight were the drift current of valley polarized electrons measured between two contacts. These experiment could then be compared with Monte Carlo simulations. The simulations gave valuable insigne into the dynamics of the electrons. This self-written code for Monte Carlo simulations is described in greater detail in this thesis. Some highlighted results of this thesis are as follows: optical observations of valley polarized diffusion, electrical control of valley polarized currents and the estimations of the acoustic deformation potentials to Du = 18.5 eV and Dd = -5.7 eV. This thesis also includes a more general part about charge transport.
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| 20. |
- Djurberg, Viktor, et al.
(författare)
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Optical detection of valley-polarized electron diffusion in diamond
- 2023
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Ingår i: Materials for Quantum Technology. - : Institute of Physics Publishing (IOPP). - 2633-4356. ; 3:2
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Tidskriftsartikel (refereegranskat)abstract
- Using the state of valley-polarization of electrons in solids is a promising new paradigm for information storage and processing. The central challenge in utilizing valley-polarization for this purpose is to develop methods for manipulating and reading out the final valley state. Here, we demonstrate optical detection of valley-polarized electrons in diamond. It is achieved by capturing images of electroluminescence from nitrogen-vacancy centers at the surface of a diamond sample that are excited by electrons drifting and diffusing through the sample. Monte Carlo simulations are performed to interpret the resulting experimental diffusion patterns. Our results give insight into the drift-diffusion of valley-polarized electrons in diamond and yield a way of analyzing the valley-polarization of ensembles of electrons.
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| 21. |
- Elamalayil Soman, Deepak, 1984-
(författare)
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Multilevel Power Converters with Smart Control for Wave Energy Conversion
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main focus of this thesis is on the power electronic converter system challenges associated with the grid integration of variable-renewable-energy (VRE) sources like wave, marine current, tidal, wind, solar etc. Wave energy conversion with grid integration is used as the key reference, considering its high energy potential to support the future clean energy requirements and due the availability of a test facility at Uppsala University. The emphasis is on the DC-link power conditioning and grid coupling of direct driven wave energy converters (DDWECs). The DDWEC reflects the random nature of its input energy to its output voltage wave shape. Thereby, it demands for intelligent power conversion techniques to facilitate the grid connection.One option is to improve and adapt an already existing, simple and reliable multilevel power converter technology, using smart control strategies. The proposed WECs to grid interconnection system consists of uncontrolled three-phase rectifiers, three-level boost converter(TLBC) or three-level buck-boost converter (TLBBC) and a three-level neutral point clamped (TLNPC) inverter. A new method for pulse delay control for the active balancing of DC-link capacitor voltages by using TLBC/TLBBC is presented. Duty-ratio and pulse delay control methods are combined for obtaining better voltage regulation at the DC-link and for achieving higher controllability range. The classic voltage balancing problem of the NPC inverter input, is solved efficiently using the above technique. A synchronous current compensator is used for the NPC inverter based grid coupling. Various results from both simulation and hardware testing show that the required power conditioning and power flow control can be obtained from the proposed multilevel multistage converter system.The entire control strategies are implemented in Xilinx Virtex 5 FPGA, inside National Instruments’ CompactRIO system using LabVIEW. A contour based dead-time harmonic analysis method for TLNPC and the possibilities of having various interconnection strategies of WEC-rectifier units to complement the power converter efforts for stabilizing the DC-link, are also presented. An advanced future AC2AC direct power converter system based on Modular multilevel converter (MMC) structure developed at Siemens AG is presented briefly to demonstrate the future trends in this area.
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| 22. |
- Engström, Jens, et al.
(författare)
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A resonant Two Body System for a point absorbing Wave Energy Converter with direct-driven linear generator
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 110:12, s. 124904-
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Tidskriftsartikel (refereegranskat)abstract
- Based on an earlier conceptual model of a two body system point absorbing wave energy converter tuned to resonance in Swedish west coast sea states, an extended coupled hydrodynamic, mechanic, and electromagnetic model has been developed. The hydrodynamic characteristics of the two body system are studied in the frequency and time domain, while its response to real Swedish west coast sea states are studied in the time domain, by using a wave energy converter model with two independently moving bodies connected to a direct driven linear generator with non-linear damping. The two body system wave energy converter gives nearly 80% power capture ratio in irregular waves. The resonant behaviour is shown to be sensitive to the shape of the spectrum, and the distance between the two bodies is shown to have a large effect on the power absorption.
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| 23. |
- Engström, Jens, 1977-
(författare)
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Hydrodynamic Modelling for a Point Absorbing Wave Energy Converter
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Surface gravity waves in the world’s oceans contain a renewable source of free power on the order of terawatts that has to this date not been commercially utilized. The division of Electricity at Uppsala University is developing a technology to harvest this energy. The technology is a point absorber type wave energy converter based on a direct-driven linear generator placed on the sea bed connected via a line to a buoy on the surface. The work in this thesis is focused mainly on the energy transport of ocean waves and on increasing the transfer of energy from the waves to the generator and load. Potential linear wave theory is used to describe the ocean waves and to derive the hydrodynamic forces that are exerted on the buoy. Expressions for the energy transport in polychromatic waves travelling over waters of finite depth are derived and extracted from measured time series of wave elevation collected at the Lysekil test site. The results are compared to existing solutions that uses the simpler deep water approximation. A Two-Body system wave energy converter model tuned to resonance in Swedish west coast sea states is developed based on the Lysekil project concept. The first indicative results are derived by using a linear resistive load. The concept is further extended by a coupled hydrodynamic and electromagnetic model with two more realistic non-linear load conditions. Results show that the use of the deep water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. Around the resonance frequency, a Two-Body System gives a power capture ratio of up to 80 percent. For more energetic sea states the power capture ratio decreases rapidly, indicating a smoother power output. The currents in the generator when using the Two-Body system is shown to be more evenly distributed compared to the conventional system, indicating a better utilization of the electrical equipment. Although the resonant nature of the system makes it sensitive to the shape of the wave spectrum, results indicate a threefold increase in annual power production compared to the conventional system.
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| 24. |
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| 25. |
- Engström, Jens, et al.
(författare)
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Performance of large arrays of point absorbing direct-driven wave energy converters
- 2013
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 114:20, s. 204502-
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Tidskriftsartikel (refereegranskat)abstract
- Future commercial installation of wave energy plants using point absorber technology will require clusters of tens up to several hundred devices, in order to reach a viable electricity production. Interconnected devices also serve the purpose of power smoothing, which is especially important for devices using direct-driven power take off. The scope of this paper is to evaluate a method to optimize wave energy farms in terms of power production, economic viability and resources. In particular, the paper deals with the power variation in a large array of point-absorbing direct-driven wave energy converters, and the smoothing effect due to the number of devices and their hydrodynamic interactions. A few array geometries are compared and 34 sea states measured at the Lysekil research site at the Swedish west coast are used in the simulations. Potential linear flow theory are used with full hydrodynamic interactions between the buoys. It is shown that the variance in power production depends crucially on the geometry of the array and the number of interacting devices, but not significantly on the energy period of the waves.
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| 26. |
- Engström, Jens, et al.
(författare)
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Properties of the Energy Transport for Plane-Parallel Polychromatic Surface Gravity Waves in Waters of Arbitrary Depth
- 2015
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Ingår i: IEEE Journal of Oceanic Engineering. - 0364-9059 .- 1558-1691. ; 40:2, s. 408-416
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Tidskriftsartikel (refereegranskat)abstract
- It is well known that the energy transport of ocean waves propagates with the group velocity and that the energy decreases exponentially with depth. Expanding this theory, we will derive expressions for the energy transport as a function of depth and the total instantaneous transport's development over time for waves in waters of finite depth. Solutions to the Laplace equation are found for plane-parallel polychromatic waves with linearized boundary conditions. A time series of wave elevation collected at Uppsala University's wave energy research test site is chosen to present the results. Solutions for waters of both infinite and arbitrary depths are presented and compared. The solutions are convolution-type integrals with the wave elevation where we have found efficient ways to calculate the kernels. The difference in group velocity between finite depth and infinite depth and its impact on the energy transport is clearly seen in the results. The use of the deep-water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. We further show that the total instantaneous energy transport can actually have a direction that is opposite to the direction of the waves as observed from a reference frame fixed to the seabed.
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| 27. |
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| 28. |
- Engström, Jens, et al.
(författare)
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Total instantaneous energy transport in polychromatic fluid gravity waves at finite depth
- 2012
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Ingår i: Journal of Renewable and Sustainable Energy. - : AIP Publishing. - 1941-7012. ; 4:3, s. 033108-1-033108-8
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Tidskriftsartikel (refereegranskat)abstract
- The total instantaneous energy transport can be found for polychromatic waves when using the deep water approximation. Expanding this theory to waves in waters of finite depth, we derive an expression for the total instantaneous energy transport for polychromatic fluid gravity waves based on potential theory with linearized free surface boundary conditions. We present the results for time series of wave elevation measured at the Uppsala University wave energy research test site. We show that a significant proportion of the total instantaneous energy transport is not accounted for when using the deep water theory. This is important since many wave energy conversion devices under development will operate in waters that do not fulfil the deep water criteria.
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| 29. |
- Engström, Jens, et al.
(författare)
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Wave energy converter with enhanced amplitude response at frequencies coinciding with Swedish west coast sea states by use of a supplementary submerged body
- 2009
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Ingår i: Journal of Applied Physics. - USA : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 106:6
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Tidskriftsartikel (refereegranskat)abstract
- The full-scale direct-driven wave energy converter developed at Uppsala University has been in offshore operation at the Swedish west coast since 2006. Earlier simulations have now been validated by full-scale experiment with good agreement. Based on that, a theoretical model for a passive system having optimum amplitude response at frequencies coinciding with Swedish west coast conditions has been developed. The amplitude response is increased by adding supplementary inertia by use of the additional mass from a submerged body. A sphere with neutral buoyancy is chosen as the submerged body and modeled as being below the motion of the waves. The model is based on potential linear wave theory and the power capture ratio is studied for real ocean wave data collected at the research test site. It is found that the power capture ratio for the two body system can be increased from 30% to 60% compared to a single body system. Increased velocity in the system also decreases the value for optimal load damping from the generator, opening up the possibility to design smaller units.
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| 30. |
- Enlund, Johannes, et al.
(författare)
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Anisotropic dry etching of boron doped single crystal CVD diamond
- 2005
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 43:9, s. 1839-1842
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Tidskriftsartikel (refereegranskat)abstract
- Semiconducting boron doped single-crystal CVD diamond has been patterned using aluminum masks and an inductively coupled plasma (ICP) etch system. For comparison insulating HPHT diamond samples were also patterned using the same process. Diamond etch rates above 200 nm/min were obtained with an O2/Ar discharge for a gas pressure of 2.5 mTorr using 600 W RF power. We have accomplished the fabrication of structures with a minimum feature size of 1 μm with vertical sidewalls in both CVD and HPHT diamond. The ICP etching produced smooth surfaces with a typical root-mean-square surface roughness of 3 nm. The dependence of etch rate on bias voltage was somewhat different for the two types of diamond. However, for all samples both the etch rate and anisotropy were found to improve with increasing bias voltage.
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| 31. |
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| 32. |
- Eriksson, Mikael, 1976-
(författare)
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Modelling and Experimental Verification of Direct Drive Wave Energy Conversion : Buoy-Generator Dynamics
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is focused on development of models and modelling of a wave energy converter in operation. Through the thesis linear potential wave theory has been used to describe the wave-buoy interaction. The differences lie in the generator models, in the simplest model the generator is a mechanical damper characterized by a damping factor. In the most advanced generator model the magnetic fields is calculated the by a FE-method, which gives detailed description of the electric properties and the effect it has on the buoy dynamics. Moreover, an equivalent circuit description of the generator has been tested. It has the same accuracy as the field based model but with a strongly enhanced CPU time. All models are verified against full scale experiments. The models are intended to be used for design of the next generation wave energy converters. Further, the developed models have also been used to study what effect buoy geometry and generator damping have on the ability to energy absorption. In the spring 2006 a full scale wave energy converter was installed at the west coast of Sweden. It was in operation and collected data during three months. During that period the load resistance was varied in order to study the effect on the energy absorption. These collected data was then used in the verification of the developed models. In the year 2002 a wave energy project started at Uppsala University; this work is a part of that larger project which intendeds to develop a viable wave energy conversion concept.
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| 33. |
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| 34. |
- Eriksson, Mikael, et al.
(författare)
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Theory and Experiment on an Elastically Moored Cylindrical Buoy
- 2006
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Ingår i: IEEE Journal of Oceanic Engineering. - 0364-9059 .- 1558-1691. ; 31:4, s. 959-963
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we compare simulated forces and accelerations for a moored floating buoy with full-scale experimental results in real ocean waves. The buoy is moored with a wire connected by springs to a concrete foundation situated at the seafloor. This study aims to develop a computer model using potential theory with a linearized free-surface boundary condition to describe the motion of such a system. The intention is to use the model for future study of wave-energy absorption and design of converters. Another objective is to see how complex a model is required to get accurate results. The method used is computationally fast and makes it possible to couple linear buoy wave interaction with nonlinear generator models, so that different loads and latching can be studied. A computationally fast method is required to model farms of wave-energy converters.
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| 35. |
- Eriksson, Mikael, et al.
(författare)
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Wave power absorption : Experiments in open sea and simulation
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 102:8, s. 084910-084910-5
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Tidskriftsartikel (refereegranskat)abstract
- A full scale prototype of a wave power plant based on a direct drive linear generator driven by a point absorber has been installed at the west coast of Sweden. In this paper, experimentally collected data of energy absorption for different electrical loads are used to verify a model of the wave power plant including the interactions of wave, buoy, generator, and external load circuit. The wave-buoy interaction is modeled with linear potential wave theory. The generator is modeled as a nonlinear mechanical damping function that is dependent on piston velocity and electric load. The results show good agreement between experiments and simulations. Potential wave theory is well suited for the modeling of a point absorber in normal operation and for the design of future converters. Moreover, the simulations are fast, which opens up for simulations of wave farms.
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| 36. |
- Gabrysch, Markus, 1978-
(författare)
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Charge Transport in Single-crystalline CVD Diamond
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Diamond is a semiconductor with many superior material properties such as high breakdown field, high saturation velocity, high carrier mobilities and the highest thermal conductivity of all materials. These extreme properties, as compared to other (wide bandgap) semiconductors, make it desirable to develop single-crystalline epitaxial diamond films for electronic device and detector applications. Future diamond devices, such as power diodes, photoconductive switches and high-frequency field effect transistors, could in principle deliver outstanding performance due to diamond's excellent intrinsic properties. However, such electronic applications put severe demands on the crystalline quality of the material. Many fundamental electronic properties of diamond are still poorly understood, which severely holds back diamond-based electronic device and detector development. This problem is largely due to incomplete knowledge of the defects in the material and due to a lack of understanding of how these defects influence transport properties. Since diamond lacks a shallow dopant that is fully thermally activated at room temperature, the conventional silicon semiconductor technology cannot be transferred to diamond devices; instead, new concepts have to be developed. Some of the more promising device concepts contain thin delta-doped layers with a very high dopant concentration, which are fully activated in conjunction with undoped (intrinsic) layers where charges are transported. Thus, it is crucial to better understand transport in high-quality undoped layers with high carrier mobilities. The focus of this doctoral thesis is therefore the study of charge transport and related electronic properties of single-crystalline plasma-deposited (SC-CVD) diamond samples, in order to improve knowledge on charge creation and transport mechanisms. Fundamental characteristics such as drift mobilities, compensation ratios and average pair-creation energy were measured. Comparing them with theoretical predictions from simulations allows for verification of these models and improvement of the diamond deposition process.
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| 37. |
- Gabrysch, Markus, et al.
(författare)
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Compensation in boron-doped CVD diamond
- 2008
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Ingår i: Physica status solidi. A, Applications and Materials Science. - : Wiley. - 1862-6300. ; 205:9, s. 2190-2194
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Tidskriftsartikel (refereegranskat)abstract
- Hall-effect measurements on single crystal boron-doped CVD diamond in the temperature interval 80-450 K are presented together with SIMS measurements of the dopant concentration. Capacitance-voltage measurements on rectifying Schottky junctions manufactured on the boron-doped structures are also presented in this context. Evaluation of the compensating donor (N-D) and acceptor concentrations (N-A) show that in certain samples very low compensation ratios (N-D/N-A below 10(-4)) have been achieved. The influence of compensating donors on majority carrier transport and the significance for diamond device performance are briefly discussed.
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| 38. |
- Gabrysch, Markus, 1978-, et al.
(författare)
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Electron and hole drift velocity in chemical vapor deposition diamond
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:6, s. 063719-
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Tidskriftsartikel (refereegranskat)abstract
- The time-of-flight technique has been used to measure the drift velocities for electrons and holes in high-purity single-crystalline CVD diamond. Measurements were made in the temperature interval 83 ≤ T ≤ 460 K and for electric fields between 90 and 4 × 103 V/cm, applied in the <100> crystallographic direction. The study includes low-field drift mobilities and is performed in the low-injection regime to perturb the applied electric field only minimally.
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| 39. |
- Gabrysch, Markus, 1978-
(författare)
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Electronic Properties of Diamond
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Diamond is a semiconductor with many superior material properties such as high breakdown field, high saturation velocity, high carrier mobilities and the highest thermal conductivity of all materials. These extreme properties compared to other (wide bandgap) semiconductors make it desirable to develop single-crystalline epitaxial diamond films for many electronic device and detector applications. Future devices, such as power diodes, photoconductive switches and high-frequency field effect transistors, could in principle deliver outstanding performance due to diamond’s excellent intrinsic properties. However, such electronic applications put severe demands on the crystalline quality of the material. Unfortunately, many fundamental electronic properties of diamond are still poorly understood, which severely holds back diamond-based electronic device and detector development. Such uncertainties are largely due to an incomplete knowledge of the types and concentrations of defects present in the material and also due to a lack of understanding of the influence that these defects have on transport properties. The focus of this licentiate thesis is therefore the study of certain electronic properties of single-crystalline plasma-deposited (SC-CVD) diamond samples in order to gain more information about the charge creation and transport mechanisms. By measuring characteristics such as drift mobilities, saturation velocities, compensation ratios or average pair-creation energy and comparing them with theoretical predictions from simulations allows for verification of these models and improvement of the diamond growth process.
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| 40. |
- Gabrysch, Markus, et al.
(författare)
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Formation of secondary electron cascades in single-crystalline plasma-deposited diamond upon exposure to femtosecond x-ray pulses
- 2008
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 103:6
-
Tidskriftsartikel (refereegranskat)abstract
- Secondary electron cascades were measured in high purity single-crystalline chemical vapor deposition (CVD) diamond, following exposure to ultrashort hard x-ray pulses (140 fs full width at half maximum, 8.9 keV energy) from the Sub-Picosecond Pulse Source at the Stanford Linear Accelerator Center. We report measurements of the pair creation energy and of drift mobility of carriers in two CVD diamond crystals. This was done for the first time using femtosecond x-ray excitation. Values for the average pair creation energy were found to be 12.17 +/- 0.57 and 11.81 +/- 0.59 eV for the two crystals, respectively. These values are in good agreement with recent theoretical predictions. The average drift mobility of carriers, obtained by the best fit to device simulations, was mu(h)= 2750 cm(2)/V s for holes and was mu(e)= 2760 cm(2) / V s for electrons. These mobility values represent lower bounds for charge mobilities due to possible polarization of the samples. The results demonstrate outstanding electric properties and the enormous potential of diamond in ultrafast x-ray detectors.
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| 41. |
- Gabrysch, Markus, et al.
(författare)
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XUV-induced transient phase gratings for probing ultra-fast carrier generation and recombination processes in wide-bandgap semiconductors
- 2013
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Ingår i: Annalen der Physik. - : Wiley. - 0003-3804 .- 1521-3889. ; 525:1-2, s. 59-65
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Tidskriftsartikel (refereegranskat)abstract
- A method for probing the temporal evolution of ultra-fast carrier generation and recombination processes in wide-bandgap semiconductors, e.g. diamond, is described. Two extreme ultraviolet (pump) pulses produced by high-order harmonic generation in Argon gas (with a photon energy of 32 eV) are superimposed on a sample with a small angle between them, inducing periodic changes in the refractive index of the material causing it to act as a transient diffraction grating. A delayed synchronized infrared (probe) pulse gets diffracted on the induced phase grating and is detected in the first diffraction order. By varying the time-delay between pump and probe, the full temporal evolution of the free carrier generation and recombination processes can be resolved. Feasibility calculations and the first steps towards experimental implementation are presented.
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| 42. |
- Giassi, Marianna, et al.
(författare)
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Capturing the experimental behaviour of a point-absorber WEC by simplified numerical models
- 2020
-
Ingår i: Journal of Fluids and Structures. - : Elsevier BV. - 0889-9746 .- 1095-8622. ; 99
-
Tidskriftsartikel (refereegranskat)abstract
- The paper presents a wave basin experiment of a direct-driven point-absorber wave energy converter moving in six degrees of freedom. The goal of the work is to study the dynamics and energy absorption of the wave energy converter, and to verify under which conditions numerical models restricted to heave can capture the behaviour of a point-absorber moving in six degrees of freedom. Several regular and irregular long-crested waves and different damping values of the power take-off system have been tested. We collected data in terms of power output, device motion in six degrees of freedom and wave elevation at different points of the wave basin. A single-body numerical model in the frequency domain and a two-body model in the time domain are used in the study. Motion instabilities due to parametric resonance observed during the experiments are discussed and analysis of the buoy motion in terms of the Mathieu instability is also presented. Our results show that the simplified models can reproduce the body dynamics of the studied converter as long as the transverse non-linear instabilities are not excited, which typically is the case in irregular waves. The performance of the more complex time domain model is able to reproduce both the buoy and PTO dynamics, while the simpler frequency domain model can only reproduce the PTO dynamics for specific cases. Finally, we show that the two-body dynamics of the studied wave energy converter affects the power absorption significantly, and that common assumptions in the numerical models, such as stiff mooring line or that the float moves only in heave, may lead to incorrect predictions for certain sea states.
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| 43. |
- Giassi, Marianna, et al.
(författare)
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Comparison of wave energy park layouts by experimental and numerical methods
- 2020
-
Ingår i: Journal of Marine Science and Engineering. - : MDPI AG. - 2077-1312. ; 8:10
-
Tidskriftsartikel (refereegranskat)abstract
- An experimental campaign of arrays with direct-driven wave energy converters of point-absorbing type is presented. The arrays consist of six identical floats, moving in six degrees of freedom, and six rotating power take-off systems, based on the design developed at Uppsala University. The goals of the work were to study and compare the performances of three different array layouts under several regular and irregular long-crested waves, and in addition, to determine whether the numerical predictions of the best performing array layouts were confirmed by experimental data. The simulations were executed with a frequency domain model restricted to heave, which is a computationally fast approach that was merged into a genetic algorithm optimization routine and used to find optimal array configurations. The results show that good agreement between experiments and simulations is achieved when the test conditions do not induce phenomena of parametric resonance, slack line and wave breaking. Specific non-linear dynamics or extensive sway motion are not captured by the used model, and in such cases the simulation predictions are not accurate, but can nevertheless be used to get an estimate of the power output.
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| 44. |
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| 45. |
- Giassi, Marianna, 1988-
(författare)
-
Numerical and experimental modelling for wave energy arrays optimization
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many wave power conversion devices, especially point-absorbers, do not provide alone the necessary amount of converted electricity to be cost effective, instead they are designed to be deployed in arrays of many units. Such arrays, or parks, can satisfy a large-scale energy demand, reduce the costs of the produced electricity and improve the reliability of the system.The performance of a wave energy park is affected by multiple and mutually interacting parameters, and the complex problem that arises during its design is called array optimization.The scope of the present thesis is to study such systems and their design, by the development of an optimization routine able to predict the best layout of a wave energy park under fixed constraints. The wave energy converter considered is the point-absorber developed at Uppsala University, which consists of a linear electric generator located on the seabed and a floating buoy at the surface.An optimization routine based on a genetic algorithm was created, which allows simultaneous optimization of the geometry of the buoys, the damping coefficient of the linear generators and the geometrical layout of the park.Finally, an experimental campaign with a single device and three arrays of six devices was conducted in order to compare the theoretical results with experimentally acquired data.The results identify optimal configurations of wave energy arrays, and highlight the effect of optimizing upon different objective functions, including economical ones. In the experiments, standard models and common assumptions used for wave energy park optimizations were tested against realistic conditions.
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| 46. |
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| 47. |
- Gravråkmo, Halvar, 1979-
(författare)
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Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wave energy converters of point absorber type have been developed and constructed. Full scale experiments have been carried out at sea and electricity has been successfully delivered. Linear permanent magnet generators together with a subsea substation and buoys of various geometric shapes have been investigated theoretically and experimentally. The design has in large extent an electronic approach, keeping the mechanical part of it as simple as possible, due to the long life span and reliability of electric components.Because of the nature of a linear generator, the internal translator with permanent magnets has a limited stroke length which will be reached when the buoy is exposed to large wave heights. Internal springs at the top and bottom of the generator prevent the translator from hitting the generator hull. Inertial forces due to the mass and velocity of the translator and the buoy and its heave added mass compresses the spring. The added mass is a rather large part of the total moving mass. Simulations of a converter with a vertical cylindrical buoy and with a toroidal buoy were conducted, as well as real sea experiments with converters with cylindrical buoys of two different sizes and a toroidal buoy. The overloads are likely to affect the design and service life of the generator, the buoy and the wire which interconnects them.Buoy shapes with as much excitation force as possible and as little heave added mass as possible were sought. A toroidal buoy caused less overloads on the generator at sea states with short wave periods and relatively large wave height, but for sea states with very long wave periods or extremely high waves, the magnitude of the overloads was mainly determined by the maximum displacement of the buoy.Snap loads on the interconnecting wire, as the slack wire tensed up after a very deep wave trough, were found to be greater but of the same order of magnitude as forces during the rest of the wave cycle.During a 4 day period at various wave conditions, two converters with cylindrical buoys proved efficiency between 11.1 % and 24.4 %. The larger buoy had 78 % larger water plane area than the other buoy which resulted in 11 % more power production. Short wave period was beneficial for the power production.Infinite frequency heave added mass was measured for a cylindrical buoy at real sea and found to be greater than the linearly calculated theoretical added mass.
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| 48. |
- Göteman, Malin, 1980-, et al.
(författare)
-
Advances and Challenges in Wave Energy Park Optimization : A Review
- 2020
-
Ingår i: Frontiers in Energy Research. - : Frontiers Media SA. - 2296-598X. ; 8
-
Forskningsöversikt (refereegranskat)abstract
- A commercial wave energy system will typically consist of many interacting wave energy converters installed in a park. The performance of the park depends on many parameters such as array layout and number of devices, and may be evaluated based on different measures such as energy absorption, electricity quality, or cost of the produced electricity. As wave energy is currently at the stage where several large-scale installations are being planned, optimizing the park performance is an active research area, with many important contributions in the past few years. Here, this research is reviewed, with a focus on identifying the current state of the art, analyzing how realistic, reliable, and relevant the methods and the results are, and outlining directions for future research.
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| 49. |
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| 50. |
- Göteman, Malin, 1980-, et al.
(författare)
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Arrays of Point-Absorbing Wave Energy Converters in Short-Crested Irregular Waves
- 2018
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Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 11:4
-
Tidskriftsartikel (refereegranskat)abstract
- For most wave energy technology concepts, large-scale electricity production and cost-efficiency require that the devices are installed together in parks. The hydrodynamical interactions between the devices will affect the total performance of the park, and the optimization of the park layout and other park design parameters is a topic of active research. Most studies have considered wave energy parks in long-crested, unidirectional waves. However, real ocean waves can be short-crested, with waves propagating simultaneously in several directions, and some studies have indicated that the wave energy park performance might change in short-crested waves. Here, theory for short-crested waves is integrated in an analytical multiple scattering method, and used to evaluate wave energy park performance in irregular, short-crested waves with different number of wave directions and directional spreading parameters. The results show that the energy absorption is comparable to the situation in long-crested waves, but that the power fluctuations are significantly lower.
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